Firestore: Small Count API documentation improvements

[dconeybe]

Apply the feedback to the count and aggregation API documentation as suggested in #6608.

Ported to Android in firebase/firebase-android-sdk#5634.
Ported to iOS in firebase/firebase-ios-sdk#12254.
Ported to CPP in firebase/firebase-cpp-sdk#1511.
Ported to Unity in firebase/firebase-unity-sdk#923.
Ported to Node server SDK in googleapis/nodejs-firestore#1974.
Ported to Java server SDK in googleapis/java-firestore#1522.

[changeset-bot]

⚠️ No Changeset found

Latest commit: b6186b0

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (d7ace80) and merge commit (3bbcaf8).

Test Logs

• Base (d7ace80): https://github.com/firebase/firebase-js-sdk/actions/runs/7478415056
• Merge (3bbcaf8): https://github.com/firebase/firebase-js-sdk/actions/runs/7478860173

1. https://storage.googleapis.com/firebase-sdk-metric-reports/5X6LtfTQi4.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (d7ace80) and merge commit (3bbcaf8).

Test Logs

• Base (d7ace80): https://github.com/firebase/firebase-js-sdk/actions/runs/7478415056
• Merge (3bbcaf8): https://github.com/firebase/firebase-js-sdk/actions/runs/7478860173

1. https://storage.googleapis.com/firebase-sdk-metric-reports/rOuF9wk7FA.html

[egilmorez]

Text edits LGTM, thanks!

[tom-andersen]

LGTM